Co-deposition of calcium carbonate and calcium sulfate scale was detected in the some of the wells in the White Tiger oilfield, but, up to now, not much attention has been paid to this problem. In fact, these salts (calcium carbonate and calcium sulfate) co-exist and co-precipitate in the system of seawater and squeeze (injected) water. The mixed CaSO4-CaCO3 scale was found to adhere to a test tube wall more strongly than the pure CaCO3 or CaSO4. More importantly, the treatment of this mixed scale in the field is much more difficult.

This study investigates the inhibition efficiency on mixed scale by using scale inhibitors such as DETPMP, EDTMP, and Lab of Magnetochemistry and Production Chemicals - Institute of Materials Science-NCST) and the chelants such as citric acid, maleic acid and ethylenediamintetraacetic acid. The experimental results show that the mixed DETPMP: MA in ratio 4:1, 1:1, and mixed DETPMP:CA in ratio 4:1; 3:1, and mixed DETPMP: EDTA (2:1) have given the high inhibition efficiency of 98.32%; 93.85%; 96.09%; 96.09%, 92.52% for mixed scale CaSO4-CaCO3 inhibition. These precipitations were analyzed by Scanning Electron Microscopy (SEM). From the SEM photos, it can be observed that for the pure CaSO4 in seawater, the crystals exhibited long needle shape structures (which is typical for calcium sulfate dihydrate) whereas the hexagonal structures were observed for the pure CaCO3 precipitant, which is a typical structure for calcite. In the mixed system, the morphology of CaSO4-CaCO3 crystals changes and spherical shape crystals are predominant. In the presence of the inhibitors, especially with the right inhibitors, the morphology of mixed scale changes and the size of crystals strongly decreases (from 100μm decreases 1-2μm) and are non-adherent. The relation between the inhibition efficiency and the scale morphology for CaSO4-CaCO3 deposits were clearly illustrated.

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